Underwater acoustic communication can be a very challenging problem due to the fundamental attributes of the underwater sound channel. These attributes include low bandwidth, long propagation delay caused by the low speed of sound in water, and noise generated by the data transmission, such as multi-path, ray bending and reflections (echoes).
If an acoustic transmitter sends data underwater to a remote node, the data radiates out in all directions (it is omni-directional) and its amplitude reduces according to the cube of its distance from the transmitter. Because the sound speed in seawater is approximately 1500 m/s, and the usable range of sending data is on the order of 2 km at a frequency 25 KHz, you have to wait after data is transmitted until all the energy propagates out of the area of interest in order to avoid interference. In the simple case of two nodes sending data back and forth at distance of 1.5 km, it will take 1 second for the first bits to arrive at the distant node. If that node sends a handshake back this will add another 1 second delay.
These attributes, especially the long propagation delays and low bit rates, make underwater acoustic networks fundamentally different from wireless packet networks. As a consequence, many of the network protocols designed for wireless packet networks are either not applicable, or have extremely low efficiency over underwater acoustic channels. Accordingly, improvements in the transfer of information over underwater acoustic networks are needed.